1. Field of the Invention
The present invention relates to a fabrication method for pattern-formed structure having a smooth three-dimensional structure.
2. Description of the Related Art
Conventionally, it has been desired to develop a blazed grating or microlens having a smooth curved surface. In reality, however, methods of producing a curved surface approximated by multistep etching are generally used. FIGS. 11A to 11H are explanatory views illustrating a conventional technique of producing a blazed grating by exposure to electron beam. As illustrated, first, a resist layer 13 is formed on a member in which a Cr layer 12 is formed on a quartz substrate 11, and the resultant is exposed to electron beam 14 into the form of a line and space pattern having a pitch “p” (FIG. 11A). Next, the resist layer 13 is developed (FIG. 11B), and the Cr layer 12 and the quartz substrate 11 are etched (FIG. 11C). The resist layer 13 is once removed (FIG. 11D). Thereafter, a resist layer 13′ is again formed on the etched quart substrate 11, and the resultant is alignment-exposed to electron beam 14′ into the form of a line and space pattern having a pitch “2p” (FIG. 11E). In the same manner, development (FIG. 11F) and etching (FIG. 11G) are performed, whereby a 4-step blazed grating can be yielded (FIG. 11H). In order to obtain a structure having steps the number of which is “2n”, it is generally necessary to perform exposure, development and etching for “n” times.
When the number of times of exposure, development and etching is increased as described above, the resultant surface can be further approximated to a curved surface. However, lenses or the like that are obtained by approximating their surface to a curved surface by multistep etching are poorer in performances in many cases than lenses having a smooth curved surface. As described above, in order to obtain steps the number of which is “2n”, it is necessary to repeat exposure, development and etching for “n” times; therefore, the following problem is caused: in order to increase the number of the steps to make the resultant surface close to a smooth curved surface, costs for the production increase. In addition, as the number of the steps becomes larger, the width of each of the steps becomes smaller; thus, there is also caused a problem that an expensive writer exhibiting a high resolution is required. It is therefore desired to develop a method making it possible to form a smooth curved surface by exposure, development and etching the repeated time number of which is smaller.
Japanese Patent Application Laid-Open (JP-A) No. 60-8844 discloses a photoetching method using electron beam in which at the time of forming a resist pattern having a substantially saw-tooth cross section by the beam irradiation, irregularities generated correspondingly to the intensity distribution of spots of the beam are made smooth. This photoetching method is a method of making three-dimensional writing directly by the electron beam. Moreover, JP-A No. 8-174563 discloses a method of forming a microscopically smooth, three-dimensional surface shape. This method is a method of using a resist in which the contrast is a “γ” value of 2.0 or less.